1. Field of the Invention
The present invention relates to a flat type cathode ray tube (CRT), and particularly, to a rail in a flat type cathode ray tube, wherein stress which occurs between a panel and a rail is reduced, thereby preventing cracks in the panel.
2. Description of the Background Art
Generally, cathode ray tube (CRT) is a device to realize a screen by deflecting an electron beam emitted from an electron gun by a deflection yoke, landing on a fluorescent surface formed at an inner surface of a panel by passing through a plurality of electron beam through holes formed on a shadow mask.
Recently, a flat type CRT having a panel of a flat type is developed and commercialized so as to prevent image distortion, to minimize a reflection by exterior light, and to maximize a visible region.
The flat type CRT will be explained with reference to attached drawings.
FIG. 1 is a schematic view showing flat type cathode ray tube (CRT) in accordance with the conventional art, and FIG. 2 is a frontal view showing a panel in a conventional flat type CRT.
As shown in FIG. 1, the conventional flat type CRT comprises a funnel 10 corresponding to a rear glass, a panel 20 corresponding to a frontal glass which is combined to the funnel 10 and sealed to be a high vacuum state. Moreover, an electron gun 40 located at an end portion of the funnel 10 for. emitting an electron beam 50; a deflection yoke 30 mounted at an outer circumference of a neck portion of the funnel 10 for deflecting the electron beam 50 towards a fluorescent surface formed at an inner side of the panel 20, a shadow mask 70 disposed at a rear surface of the panel 20 for sorting out colors of the electron beam 50, and an inner shield 60 prolonged from the panel 20 to the funnel 10 for shielding an external terrestrial magnetism.
Also, a fluorescent film 22 playing a role of luminescent material is deposited in the panel 20, and a rail 80 is attached to the panel to fix the shadow mask 70 and the inner shield 60.
The rail 80, as shown in FIG. 2, includes two major side rails 86 and two minor side rails 87, wherein the major and minor side rails 86 and 87 have different lengths and a same shape, and end portions thereof are cut with 45° respectively and engaged to each other with forming a corner portion 88.
At this time, the corner portion 88 is grinded with a predetermined curvature so as to prevent stress from being concentrated.
The rail 80 is fixed to the panel 20 by a frit glass, wherein one side thereof is formed as a stair shape to fix the shadow mask 70 and the inner shield 60, and the other side thereof is formed as a plane shape to be attached to the panel 20.
A process for attaching the rail 80 to the panel 20 will be explained with reference to FIG. 3.
FIG. 3 is a sectional view showing a process for fixing the rail 80 of the flat type CRT to the panel 20.
First, a frit glass 90 is deposited on a panel fusion portion 84 of the rail 80 in a furnace of high temperature of 400° C.˜500° C.
Then, the rail 80 on which the frit glass 90 is deposited is attached to the panel surface 20, and passes through the furnace of high temperature of 400° C.˜500° C. According to this, the rail 80 is attached to the panel 20 by melting of the frit glass 90, thereby completing the attachment.
When the rail 80 is attached to the panel 20 at high temperature by the frit glass 90, the rail 80 and the panel 20 have thermal expansion and contraction through the furnace process. At this time, since the rail 80 and the panel 20 have different coefficients of thermal expansion and contraction, remained thermal stress exists by the difference. According to this, cracks occur in the panel 20 due to the thermal stress, thereby damaging the panel 20.
A distribution of the thermal stress occurring at the panel 20 will be explained with reference to the attached drawings.
FIG. 4 is a graph showing a change of stress existing on the panel to which the rail in the conventional flat type CRT is attached according to a location change from a center of the panel to a center of the minor side rail 87.
As shown in FIG. 4, stress is increased towards an outer side of the minor side rails (X axis) from a center portion (point O) of the panel 20, and tensile stress and compression stress are crossed at the fusion portion on which the rail 80 and the panel 20 are fixed.
At this time, cracks occur at the fusion portion due to unbalance of the stress, and the stress unbalance phenomenon severely occurs at the corner portion 88 of the rail 80.
The said phenomenon is resulted from the different thermal expansion ratio between the rail 80 and the panel 20, resulted from that the rail 80 is in contact with the panel 20 to cause the stress unbalance severely, or resulted from that the frit glass 90 for buffering the stress is not sufficiently inserted between the rail 80 and the panel 20.
In the meantime, as another example of the conventional flat type CRT, a receiving groove 102 for receiving the frit glass 90 is formed in the rail 80 so as to buffer the stress occurring by the different thermal expansion ratio between the rail 80 and the panel 20 more efficiently by sufficiently disposing the frit glass 90 between the rail 80 and the panel 20.
That is, as shown in FIGS. 5A and 5B, the receiving groove 102 for receiving the frit glass 90 is formed with a half circle or a square sectional shape in a longitudinal direction of the rail 100.
Accordingly, the stress is buffered more efficiently by disposing the frit glass 90 in the receiving groove 102, and the stress due to the different thermal expansion ratio between the rail 100 and the panel 20 is more reduced by reducing a contacted area between the rail 100 and the panel 20.
However, as shown in FIG. 6, since the receiving groove 102 is formed, in case of that the corner portion 88 of the rail, that is, a portion where the major side rail 86 and the minor side rail 87 are engaged, has a grinding with a predetermined curvature, the receiving groove 102 is opened towards an outer side of the corner portion 88, so that the frit glass 90 is leaked to the opened portion of the corner portion 88 and the leaked frit glass 90 causes thermal stress to be concentrated at the corner portion 88, thereby causing cracks in the panel 20.